How to tell a shrub from a tree: A life‐history perspective from a South African savanna

Abstract

The ecological differences between 'shrubs' and 'trees' are surprisingly poorly understood and clear ecological definitions of these two constructs do not exist. It is not clear whether a shrub is simply a small tree or whether shrubs represent a distinct life-history strategy. This question is of special interest in African savannas, where shrubs and trees often co-dominate, but are often treated uniformly as 'woody plants' even though the tree to shrub ratio is an important determinant of ecosystem functioning. In this study we use data from a long-term fire experiment, together with a trait-based approach to test (i) if woody species usually classified as shrubs or trees in African savanna differ in key traits related to disturbance and resource use; and (ii) if these differences justify the interpretation of the two growth forms as distinct life-history strategies. We measured for 22 of the most common woody plant species of a South African savanna 27 plant traits related to plant architecture, life-history, leaf characteristics, photosynthesis and resprouting capacity. Furthermore we evaluated their performance during a long-term fire experiment. We found that woody plants authors call (i) shrubs; (ii) shrubs sometimes small trees; and (3) trees responded differently to long-term fire treatments. We additionally found significant differences in architecture, diameter-height-allometry, foliage density, resprouting vigour after fire, minimum fruiting height and foliar δ 13 C between these three woody plant types. We interpret these findings as evidence for at least two different life-history-strategies: an avoidance/adaptation strategy for shrubs (early reproduction + adaptation to minor disturbance) and an escape strategy for trees (promoted investment in height growth + delayed reproduction).

Developing allometric models to predict the individual aboveground biomass of shrubs worldwide

“…Otherwise, we assumed the mean value for the species reported in the global wood density database (Chave et al, 2009), as well as in other references (see Supporting Information To test if model parameters change under particular bioclimatic conditions, we included the species' bioform, as well as the corresponding biome and the global aridity index (GAI) category for the study site. We subcategorized shrub species' bioforms as mangroves, subshrubs, shrubs (i.e., small size woody individuals typically multistemmed) and "shrubs sometimes small trees" (SST) (i.e., medium sized woody plants, with variable architecture from multistemmed to single-stemmed) (Zizka, Govender, & Higgins, 2014). This categorization followed the authors' description of the species and the available information on local floras or digital repositories (see specific references in Supporting Information Text S1).…”

Future of African terrestrial biodiversity and ecosystems under anthropogenic climate change

“…However, CO 2 -accelerated growth and spread of woody invasive alien species that avoid nutrient limitation via nitrogen fixation cannot be excluded. Unfortunately, DGVM approaches are not well developed for these vegetation types due to inadequate treatment of the shrub functional type 90 , and the incomplete incorporation of soil nutrient feedback control on production. Better-developed mechanistic modelling approaches would allow the implications of these trends on vegetation structure, function and biodiversity to be explored.…”

African shrub distribution emerges via a trade‐off between height and sapwood conductivity

“…The interest for understanding the ecology and biogeography of shrubs is growing (Scheffer et al., ), yet the distinction between shrub and tree growth forms is not trivial. One difference between shrubs and trees is that trees are typically single‐stemmed while shrubs are typically multistemmed (Zizka, Govender, & Higgins, ). This difference implies a fundamental trade‐off (Götmark et al., ; Midgley, ).…”

“…The multistemmed architecture means, for a given biomass, shorter individuals. Thus, shrubs are typically smaller than trees and seldom taller than 3–6 m (Götmark et al., ; Zizka et al., ). However, for a given biomass, shrubs have relatively larger total stem cross‐section area and therefore higher sapwood area and higher hydraulic conductivity (Götmark et al., ).…”

“…Many shrub species can cope with fire due to their ability to resprout efficiently after being damaged by fire (Hoffmann & Solbrig, ) or by regrowth from a persistent seed bank in systems where crown fires are frequent (Pausas, Bradstock, Keith, & Keeley, ). In contrast, trees grow taller than shrubs and typically cope with fire by escaping the flame zone and thereby becoming fire resistant (Hoffmann et al., ; Zizka et al., ). Tree height growth also implies both a competitive advantage for trees in dense ecosystems where light is limiting and defence against herbivores by keeping leaves away from browsers.…”